Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. Tang et al. (Applied Physics Letters, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly assigned U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved. FIG. 1 illustrates schematically the cross-sectional view of a prior art bottom-emitting OLED. Prior art bottom-emitting OLED device 101 includes a substrate 10, a transparent bottom-electrode 12a, an organic EL element 14, and a reflective metallic top-electrode 16R. The organic EL element 14 can include one or more sub-layers including a hole-injection layer (HIL) 14a (not shown), a hole transport layer (HTL) 14b, a light-emitting layer (LEL) 14c, an electron transport layer (ETL) 14d, and an electron-injection layer (EIL) 14e (not shown). In FIG. 1 the transparent bottom-electrode 12a is the anode and the reflective metallic top-electrode 16R is the cathode; but the reverse can also be the case and if so the order of the sub-layers in the organic EL element 14 is reversed.
One of the most promising applications for OLED devices is to use them in color organic light-emitting displays. A color organic light-emitting display is a device comprising more than one area or pixel that emits more than one color. When the size of the individual areas is relatively large and the number of individual areas is small, the display is generally referred to as an area color display. When the size of the individual areas is small but the number is large, the display is generally referred to as a pixelated display. The latter is the preferred device for applications that need to display high-resolution full color images. Most commonly, the pixels are divided into three different color pixel sets each of which emits a primary color of blue, green, or red. By applying different combinations of powers to drive the pixels, a full color image can be displayed.
Several different methods have been attempted to provide the different colored pixels. The most direct way is to dispose different colored emitting layers to different pixels. This can be done for small molecule OLED devices by using shadow masks during the vapor deposition process to selectively deposit the different colored emitter materials to different pixels. Although this method has been demonstrated to fabricate high performance devices, the process is complicated when high resolution pixels are required. The shadow masks are expensive and the alignment of masks is difficult. An alternative method that has been proposed is to use close-spaced vapor transport using a scanning laser beam. Although the use of shadow masks can be eliminated, the equipment is complicated and high quality devices have not been reported. Another method that is particularly suitable for fabricating polymer based OLED devices is to use ink-jet printing to deposit the emitter materials at the desired pixel locations. Although the method is attractive, it has been difficult to fabricate high performance, stable devices.
A different approach is to use a common emitter for all pixels and then create different colors using other means. For example, a white emitting OLED can be used in combination with color filters for different pixels to generate different colors. The major drawback, in addition to the cost of providing the color filters, is the loss of about two-thirds of light by the absorption of the filters. An alternative approach is to use a blue emitting OLED in combination with different florescent materials for different pixels to generate different colors. Since blue emitting OLED devices are generally less stable and less efficient than other OLEDs, this method has fundamental deficiencies.
Yet another proposed approach is to use a microcavity for pixelation. In a microcavity OLED device (U.S. Pat. Nos. 6,406,801 B1; 5,780,174 A1, and JP 11,288,786 A) the organic EL element is disposed between two highly reflecting mirrors, one of which is semitransparent. The reflecting mirrors form a Fabry-Perot microcavity that strongly modifies the emission properties of the organic EL element disposed in the cavity. Emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced through the semitransparent mirror and emission at wavelengths away from the resonance is suppressed. The use of microcavity in OLED devices has been shown to reduce the emission bandwidth and improve the color purity of emission (U.S. Pat. No. 6,326,224). There also have been suggestions that the luminance output could be enhanced by the use of microcavity [Yokoyama, Science, Vol. 256 (1992) p66; Jordan et al Appl. Phys. Lett. 69, (1996) p1997]. There have been proposals to achieve pixelation by using the color selectivity of the microcavities. The proposal was to provide a broadband emitter and to provide different microcavity length for different pixels (U.S. Pat. Nos. 5,554,911; 5,847,506; 5,969,475; and 5,405,710). Although the proposal has merits, the practical application of the proposed concepts has been difficult because in these and other prior art disclosures on microcavity OLED devices at least one of the reflecting mirrors is a Quarter Wave Stack (QWS). A QWS is a multi-layer stack of alternating high index and low index dielectric thin-films, each one a quarter wavelength thick. It can be tuned to have high reflectance, low transmittance, and low absorption over a desired range of wavelength.
FIG. 2 illustrates schematically the cross-sectional view of an exemplary prior art QWS based microcavity OLED device 102. QWS based microcavity OLED device 102 includes a substrate 10, a QWS reflecting mirror 18 as a semitransparent reflector, a transparent bottom-electrode 12a, an organic EL element 14, and a reflective metallic top-electrode 16R. A typical QWS reflecting mirror 18 is of the form TiO2:SiO2:TiO2:SiO2:TiO2 with TiO2 n=2.45 and SiO2 n=1.5 [as in R. H. Jordan et al., Appl. Phys. Lett 69, 1997 (1996)]. Thickness of each material is 56 nm and 92 nm, respectively, corresponding to quarter wavelength for green emission at 550 nm. In operation only a narrow band light centered at the resonance wavelength of 550 nm is emitted through the QWS layer out of the microcavity OLED device.
A QWS is complicated in structure and expensive to fabricate. The resonance bandwidth is extremely narrow and, as a result, even though a microcavity based on a QWS is capable of greatly increasing the emission peak height at the resonance wavelength, the total luminance integrated over wavelength is much less improved and can actually decrease over a similar device without the microcavity. In addition, the dielectric layers are not electrically conductive. To form a functional OLED device, a separate transparent conductive electrode layer needs to be disposed between the QWS and the organic layers. This added conductive electrode layer further complicates the structure. If a transparent conductive oxide is used as the conductive electrode, the electrical conductance is limited and can be inadequate for many devices especially those having large areas. If a thin metal film is used, the cavity structure is much more complicated and device performance can be compromised. QWS-based microcavity OLED devices are therefore not suitable for practical color organic light-emitting displays.
It is generally believed, however, that a QWS constructed of non-absorbing dielectric materials is necessary in achieving useful microcavity effects. Yokoyama et al [Science V256, p 66 (1992)] in his well-referenced review article specifically recommended the use of a QWS instead of metallic mirrors. Published attempts to replace the QWS have not been very successful. Berggrem et al. [Synthetic Metals 76 (1996) 121] studied a PLED using an Al mirror and a Ca—Al semitransparent mirror to construct a microcavity. Although some bandwidth narrowing was observed suggesting microcavity effect, the external quantum efficiency of the device with microcavity was a factor of three less than a similar device without microcavity. Takada et al [Appl. Phys. Lett. 63, 2032 (1993)] constructed a microcavity OLED device using a semitransparent (36 nm) Ag cathode and a 250 nm MgAg anode. Although angular distribution change and emission bandwidth reduction were observed, the emission intensity was significantly reduced compared with a non-cavity case. The authors concluded that the combination of emission dyes with broad emission spectra and a simple planar cavity was not satisfactory for the confinement of light in the microcavity, and encouraged development of new cavity structures. Jean el al [Appl. Phys. Lett., Vol 81, (2002) 1717] studied an OLED structure using a 100 nm Al as the anode and 30 nm Al as the semitransparent cathode to construct a microcavity structure. Although a strong microcavity-effect-caused emission bandwidth narrowing and strong angular dependence was observed, no improvement in emission output efficiency was suggested. In fact, judging from the extremely narrow emission bandwidth of the devices, the emission output efficiency was most likely decreased. EP 1,154,676, A1 disclosed an organic EL device having a bottom-electrode of a light reflective material, an organic light-emitting layer, a semitransparent reflection layer, and a top-electrode of a transparent material forming a cavity structure. The objective was to achieve an OLED device with sufficient color reproduction range over a wide viewing angle. The objective was achieved by reducing the microcavity effect to achieve a large emission bandwidth. Although it alleged that multiple reflection enhances resonance wavelength emission, no actual or simulated data supported the suggestion. All examples used a Cr reflective anode. The evidence shows that little luminance enhancement is achievable when an anode with reflectivity as low as Cr is used.
Lu et al. (Appl. Phys. Lett. 81, 3921 (2002)) described top-emitting OLED devices that the authors alleged to have performance enhanced by microcavity effects. However, their performance data showed very little angular dependence characteristic of microcavities. Although no spectral data were shown, the similarity in color coordinates between their non-cavity bottom-emitting structure and microcavity top-emitting structure suggests that the bandwidth narrowing effect expected in microcavity OLED devices is most likely absent as well. Indeed, our model calculations confirm that their structure should not produce a significant microcavity effect. Thus, the observed emission enhancement is most likely a result of normal modest optical interference effects typically seen in non-microcavity OLED devices. The magnitude of the emission enhancement is very small and the color quality improvement is absent. The authors also suggested that the best efficiency is achieved by using a high reflectivity anode and a transparent cathode, the latter being clearly contrary to the teaching of the present invention.